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Abstract:

The invention relates to a reproduction device (21), with a device (20)
acting as a source of digital services. It also relates to a method of
synchronizing two parts of a digital service in a system including a
source device according to the invention and at least one reproduction
device according to the invention.
According to the invention, the reproduction device (21) includes means
for receiving the data forming at least a part of a digital service
originating from a digital service source device (20), means for
processing (210) at least some of the data received, means (211) for
reproducing an output of at least a part of the digital service, the time
for processing and reproducing the data introducing a delay in the output
of the reproduced data. This device also includes communication means
(213) for informing the source device of the delay introduced.

Claims:

1. Device for reproducing at least one part of data corresponding to at
least one digital service originating from a digital service source
device comprising a first receiver for receiving first data forming a
first part of the digital service, a first processing unit for processing
said first received data, a first reproduction unit for reproducing an
output of said first processed data, the time for processing and/or
reproducing the first received data introducing a first delay-in the
output of the first reproduced data, wherein said reproduction device
further comprises a second receiver for receiving second data forming a
second part of the digital service, a second processing unit for
processing said second received data, a second reproduction unit for
reproducing an output of said second processed data, the time for
processing and/or reproducing the second data introducing a second delay
in the output of the second reproduced data and wherein the device also
comprises a communication unit for informing the source device of said
first delay and said second delay, wherein said first and second delays
vary according to the format of the first data.

2. Device according to claim 1, wherein said first received data being
video data, said reproduction device further comprises a storage unit for
storing at least two predefined values of said first delay, each of said
values relating to a predefined video data format.

3. Device according to claim 1, wherein said second received data being
audio data, said storage unit stores at least two predefined values of
said second delay, each of said values relating to a predefined video
data format.

4. Device acting as a digital service source, comprising a unit for
outputting first data forming a first part of a digital service, a unit
for outputting second data forming a second part of the digital service,
wherein it also comprises a unit for receiving from a device for
reproducing at least said first data a first delay indication relating to
said first data and a second delay indication relating to said second
data, a delay unit for applying a programmable delay to the output data
forming the second part of the digital service according to said first
and second delay indications received, wherein said first delay and
second delays vary according to the format of the first data.

5. Device according to claim 4, wherein said delay unit applies a
programmable delay to the output data forming the second part of the
digital service equals to the difference between said first delay
indication received and said second delay indication received.

6. Method for reproducing at least one part of data corresponding to at
least one digital service comprising the steps of: receiving first data
forming a first part of a digital service; processing said first received
data; reproducing an output of said first processed data, the time for
processing and/or reproducing the first received data introducing a first
delay in the output of the first reproduced data, wherein said
reproduction method further comprises: receiving second data forming a
second part of the digital service; processing said second received data;
reproducing an output of said second processed data, the time for
processing and/or reproducing the second received data introducing a
second delay in the output of the second reproduced data, transmitting
said first and second delays, wherein said first and second delays vary
according to the format of the first data.

7. Method for transmitting data corresponding to at least one digital
service, comprising the steps of: outputting first data forming a first
part of the digital service; outputting second data forming a second part
of the digital service; wherein said method further comprises the steps
of: receiving a first delay indication relating to a delay introduced
when processing and/or reproducing said first data; receiving a second
delay indication relating to a delay introduced when processing and/or
reproducing said second data; and applying a programmable delay to the
output data forming the second part of the digital service according to
said first and second delay indications received, wherein said first and
second delays vary according to the format of the first data.

8. Method according to claim 7, wherein the programmable delay equals to
the difference between said first delay indication received and said
second delay indication received.

Description:

1. FIELD OF THE INVENTION

[0001] The invention relates to a device and method for synchronizing
different parts of a digital service. The invention may, for example,
relate to the audio/video synchronization of an audiovisual digital
service.

2. DESCRIPTION OF THE PRIOR ART

[0002] For many years, screen-related technologies were based on cathode
ray tube screens. These technologies were then purely analogue. Since the
1990s, digital technologies have become more and more prevalent in the
image system from acquisition by the camera of the video signal through
to its display on the screens (for example, 100 Hz screens using motion
compensation). Initially, none of these new technologies introduced any
meaningful delay on the video. The audio/video (hereinafter denoted A/V)
synchronization is performed by the decoder, based on the assumption that
the audio and video streams supplied by the decoder are reproduced by the
audiovisual reproduction device instantaneously. In the case of the
decoders, the A/V synchronization principle consists in using time
markers ("Program Clock References" and "Presentation Time Stamps")
embedded by the MPEG encoder in the audio and video packets, enabling the
decoder to present the video and audio relative to a common time
reference. Appendix D of the ISO/IEC 13818-1 standard describes in detail
how to perform this A/V synchronization (called "LIPSYNC"). Today, the
procedure for tuning the A/V synchronization module of a decoder consists
in decoding audio and video packets derived from a test MPEG stream and
presenting them to a reproduction device (for example a CRT television)
for which the response time is considered to be instantaneous. Similarly,
in the case of DVD players, the A/V synchronization is handled by the
player itself which ensures the synchronization of the audio and video
streams at the output of the player.

[0003] Through recent advances in screen technology, a range of new
screens and more or less complex audiovisual reproduction devices have
been able to be marketed, including:

[0004] Audiovisual reproduction
devices ("home cinema" for example), in which the audio signal can be
supplied in a decoded form (PCM--pulse-code modulation--format) or in an
encoded form (for example, Dolby Digital).

[0005] High definition
(hereinafter denoted HD) television which is becoming more and more
popular in certain countries. It could become a consumer market with, for
example, the MPEG-4 technology to reduce the costs. A number of HD
formats ought to coexist with the standard definition (hereinafter
denoted SD) format. The HD format requires significant video processing
in the screen before displaying the video, which can introduce delays.

[0006] Numerous screen technologies (for example LCD, LCOS, DLP, Plasma,
etc) are available on the market for both HD and SD formats. These
various screens require their own video processors for optimal rendition,
and can thus introduce delays.

[0007] In the past, studies on the audiovisual system have shown that the
human being is sensitive to A/V phase shifts. The study carried out by
Bell laboratories in 1940 thus showed that difficulties arise with an
audio delay greater than 100 ms or an audio advance greater than 35 ms.
In practice, the human being is naturally more tolerant to an audio delay
than to an advance because it is not natural to hear a sound of an event
before seeing it displayed on the screen. Consequently, and to have
common rules, the ITU standardized the acceptable and unacceptable A/V
synchronization errors throughout the A/V system. In 1993, the
ITU[DOC11/59] standard defined the detectability range as being an audio
delay greater than 100 ms or an audio advance greater than 20 ms. The
objectionability range is defined as being an audio delay greater than
160 ms or an audio advance greater than 40 ms. In 1998, for no particular
reason, the ITU relaxed the detectability range to an audio delay greater
than 125 ms or an audio advance greater than 45 ms. The objectionability
range is then defined as being an audio delay greater than 185 ms or an
audio advance greater than 90 ms. These ranges are defined by the ITU-R
BT 1359-1 standard.

[0008] Today, the ATSC ("Advanced Television System Committee", an
international organization for developing digital television standards)
indicates that this standard is not suitable and does not conform to the
study carried out by BELL. It therefore proposes to standardize the
synchronization errors within the range [-90 ms, +30 ms] to be
distributed over the A/V system as follows: [-45 ms, +15 ms] for
acquisition and [-45 ms, +15 ms] for the encoder/decoder/TV.

[0009] Today, video reproduction devices (for example, LCD screens)
introduce delays measured in tens of milliseconds (often nearly a
hundred) in the video processing system. The delay introduced can vary
significantly from one device to another, and it can also vary according
to the format of the image which can be interlaced (for example 576i25
for SD or 1080i25 for HD) or progressive (for example 576p25 for SD or
720p50 for HD), particularly when the screen is fitted with a
deinterlacing function. These processes require the use of image memories
(for example, FIFOs, SDRAM, etc) which consequently increase the delays
on the video signal compared to the audio signal. This means that an
audio signal often precedes the video signal with which it is associated.
In practice, for their part, the audio reproduction devices do not
usually introduce a significant delay in normal use. They can introduce
delays if sound effects are added. However, these delays remain tolerable
to the user.

[0010] Unlike cathode ray tube screens, the new flat screens currently
used do not therefore respond instantaneously. In practice, their various
component modules introduce delays. FIG. 1 shows, in the form of blocks
10, 11 and 12, certain modules of an audiovisual reproduction device 1
(for example, a flat screen television) according to the state of the
art. This device includes a video reproduction device 12 (for example a
screen) and an audio reproduction device 13 (for example, an external or
built-in speaker). The conventional modules of the video reproduction
device (for example, a tuner, a PAL decoder and A/D converters) will not
be described any more. The modules 10, 11 and 12 introduce video delays
which can be fixed or variable from frame to frame. These delays vary
according to the processing applied and according to the screen type. If
they are not compensated, they will provoke A/V synchronization errors
that can be detected by the users because they are situated outside the
tolerance range as defined previously.

[0011] The first deinterlacing and format control module 10 converts an
interlaced video into a progressive video and adjusts the resolution of
the input signal to that of the screen (for example, switching from
1920×1080i to 1280×720p). This block uses a frame memory
(SDRAM, DDRAM) which introduces a variable delay (Dd) according to
the video format (interlaced/progressive, 50 Hz/60 Hz).

[0012] The second screen controller module 11 converts a progressive video
into a compatible format for the screen. The controller addresses the
screen and also performs image quality enhancement processes. These often
introduce delays Dc which depend on the type of screen.

Thus, in the case of LCD-LCOS screen (LCD standing for "Liquid Crystal
Display" and LCOS standing for "Liquid Crystal on Silicon"), it is
possible to apply the following processes which introduce delays:

[0013] Overdriving, to enhance the response times of the liquid crystal
when it needs to switch from one grey level to another. This operation
which uses a frame memory introduces a fixed delay Rc_lcd_overdriving.

[0014] The frame duplication in the three-valve LCOS systems is routinely
used to reduce the large area flicker effects. This operation, which uses
a frame memory, introduces a fixed delay Rc_lcos_double.

[0015] The conversion to sequential colour performed using a frame
memory which introduces a fixed delay Rc_dlp-lcos sequential.

[0016] The
DLP screen addressing performed by successive bit planes. This operation
introduces a fixed delay Rc-dlp_bitplane. For plasma screens, the
following processes and operations introduce delays due to:

[0017] The
screen addressing by successive subscanning operations. This introduces a
delay Rc_plasma_bitplane.

[0018] The motion compensation to reduce false
contour effects and blurring effects. This operation, which uses a frame
memory, introduces a fixed delay Rc_plasma_artefact. Similarly, the OLED
(Organic Light Emitting Diode) screens can introduce delays. The third
module 12 comprises the screen itself. The light emitted by the LCD/LCOS
screen is obtained by modulating the voltage applied to the liquid
crystal. In the case of a DMD® (Digital Micro-mirror Device), the
light is binary-modulated using pivoting micro-mirrors. In the case of a
PLASMA panel, the light is also binary-modulated by gas excitation. The
light therefore reacts with a delay relative to the modulation. This
delay depends mainly on the physical properties of the components of the
screen (liquid crystal, gas, etc). Furthermore, some screens also
incorporate an internal memory (DLP-LCOS sequential) which provokes an
additional delay. The screen therefore introduces delays De directly
linked to its type. Thus, the LCD-LCOS screens introduce, among other
things, the following delays:

[0019] Since the screen is addressed row by
row, the last row is refreshed one frame period after the first row. This
addressing operation introduces a fixed delay Re_lcd_addressing.

[0020]
The liquid crystal takes a certain time to be set up after application of
the modulation voltage. This time is broken down into a delay and a setup
time. These 2 times depend on the grey transition level between the
preceding frame and the current frame. These two times are added together
to give a variable delay Re_lcd_liquid-crystal. Other screen types (for
example plasma panels, DLP, OLED) can introduce other delay types. Thus,
the plasma screens introduce, among other things, the following delay:

[0021] The gas enclosed in the screen has a response time which varies
according to the video content and which therefore corresponds to a
variable delay Re_plasma_gas. The DLP® screens introduce in particular
the following delays:

[0022] The display device contains an internal
memory and is addressed in subscanning form. This introduces a fixed
delay Re_dlp_addressing.

[0023] The DMD® has very fast response times.
It introduces no particular delay. The table below summarizes examples of
various delay types for different screens. In the table, T represents the
frame period (20 ms/50 Hz, 16.7 ms/60 Hz).

[0024] Depending on the screen technologies used, it is therefore possible
to have delays on the video that are more or less significant, fixed or
variable from frame to frame according to the content of the image (for
example the grey levels). These delays can also vary according to the
video format. In the case of television or DVDs, there are four possible
formats:

[0025] 50 Hz interlaced input;

[0026] 50 Hz progressive input;

[0027] 60 Hz interlaced input;

[0028] 60 Hz progressive input. These
delays between the audio and video streams also depend on the audio
format that is used (for example, MPEG1, MPEG2 layer 1 and 2, DOLBY
AC-3). They can provoke out-of-tolerance A/V synchronization errors (in
other words errors beyond the tolerance range) that can be extremely
objectionable to the user. The above analysis shows that it is therefore
necessary to synchronize the A/V streams in order to improve the
perception comfort of the user and to keep the delay (or advance) in the
reproduction of the video stream relative to the audio stream within the
tolerance range defined by the standards. More generally, it is necessary
to synchronize the various parts of a digital service in order to keep
the delay (or advance) in the reproduction of one of the parts of the
service relative to the other within a tolerance range for this delay (or
this advance) not to be objectionable to the user.

3. SUMMARY OF THE INVENTION

[0029] The object of the invention is to overcome these drawbacks in the
prior art. To this end, the present invention proposes a device and a
method of synchronizing a number of parts of a digital service which take
into account delays introduced by the various processes applied to at
least a part of the digital service and delays introduced by the
reproduction devices themselves. The aim is to avoid departing from the
tolerance ranges which would be objectionable to the user.

[0030] To this end, the invention proposes a device for reproducing data
corresponding to at least one digital service including means for
receiving data forming at least a part of a digital service originating
from a digital service source device, means for processing at least a
part of the data received, means for reproducing an output of at least a
part of the digital service, the time for processing and reproducing the
data introducing a delay in the output of the reproduced data. According
to the invention, the reproduction device also includes communication
means for informing the source device of the delay introduced.

[0031] According to a preferred embodiment, the reproduction device is a
television, the digital service is an audiovisual service, and the
processed data is video data organized in frames. Moreover, one of the
means for reproducing an output of at least a part of the digital service
is a screen, preferably a flat screen such as a liquid crystal display
(LCD) screen, a plasma screen, an OLED screen or a DLP screen.

[0032] According to a particular characteristic, one of the means for
processing at least a part of the data received is a deinterlacer.

[0033] Advantageously, a value for the delay is stored in a non-volatile
memory of the reproduction device. According to a particular
characteristic, the non-volatile memory is an EPROM memory.

[0034] According to a preferred embodiment, a value of the delay is
presented in the form of an EDID descriptor.

[0035] Preferably, the communication means for informing the source device
of the delay introduced include a link using the DDC protocol or the CEC
protocol. The decoder recovers the delay value stored in EDID descriptor
form via a DDC link.

[0036] The invention also relates to a device acting as a digital service
source, including means for outputting data forming a first part of a
digital service, second means for outputting the data forming a second
part of the digital service, and means for communicating with a device
for reproducing the data forming the first part of the digital service.
The source device also includes means for applying a programmable delay
to the output data forming the second part of the digital service, means
for receiving from the device for reproducing the data forming the first
part of the digital service a delay indication and means for programming
the means for applying a programmable delay according to the delay
indication received.

[0037] According to a particular embodiment, the device acting as a source
of digital services is a digital decoder. According to another
embodiment, the device acting as a source of digital services is a DVD
player.

[0038] According to a preferred embodiment, the data forming the first
part of the digital service is video data and the data forming the second
part of the digital service is audio data.

[0039] According to another embodiment, the data forming the first part
and the second part of the digital service is video data.

[0040] Preferably, the means for applying a programmable delay compensate
for a delay due to one or more following elements of the reproduction
means:

[0041] a module for deinterlacing the video data (10);

[0042] a
format controller (10);

[0043] a screen controller (11);

[0044] a screen
(12).

[0045] According to a particular characteristic, the means for applying a
programmable delay contain a memory which temporarily stores the data
forming the second part of the digital service before restoring it
according to the delay indication received.

[0046] Finally, the invention relates to a method of synchronizing two
parts of a digital service in a system including a source device and at
least one reproduction device, in which the source device includes first
means for outputting the data forming the first part of the digital
service, second means for outputting the data forming the second part of
the digital service, means for communicating with the device for
reproducing the data forming the first part of the digital service, means
for applying a programmable delay to the output data forming the second
part of the digital service,

and in which the reproduction device includes means for receiving the
data forming at least a first part of the digital service originating
from the digital service source device, means for processing at least a
part of the data received to reproduce at least a part of the digital
service, including the following steps:

[0047] on the reproduction
device side, transmitting to the source device the total delay introduced
by the reproduction device when processing and reproducing the received
data forming at least a first part of the digital service; and

[0048] on
the source device side, programming the programmable delay, using the
delay indications received, to delay the output of the data forming the
second part of the digital service.

[0049] According to a particular embodiment, a part of the delay is due to
the characteristics of the screen and can be estimated for each frame in
the case of liquid crystal screens according to the following steps:

[0050] Computation for each pixel of the grey level difference between
two successive frames;

[0051] Estimation for each pixel of the response
time between said two successive frames from said grey level difference
computed for said pixel;

[0052] Creation of a histogram of the delays on
all of the pixels;

[0053] Computation of an average delay from said
histogram.

[0054] According to a particular characteristic, the data forming the
first part of the digital service is video data and the data forming the
second part of the digital service is audio data.

[0055] According to another characteristic, the data forming the first
part of the digital service and the second part of the digital service is
video data.

4. DESCRIPTION OF THE DRAWINGS

[0056] The invention will be better understood and illustrated by means of
advantageous exemplary embodiments, by no means limiting, with reference
to the appended figures in which:

[0057] FIG. 1, already described, shows a diagram of a flat screen
television according to the state of the art;

[0058] FIG. 2 represents a device for receiving and reproducing an
audio/video digital service according to the invention which uses an
internal audio reproduction device;

[0059]FIG. 3 represents a device for receiving and reproducing an
audio/video digital service according to the invention which uses an
external audio reproduction device;

[0060]FIG. 4 represents a device for receiving and reproducing an
audio/video digital service according to the invention in which the
reproduction device estimates a delay compensated in the source;

[0061]FIG. 5 represents a device for receiving and reproducing an
audio/video digital service according to the invention in which the
reception device estimates and compensates for a delay;

[0062]FIG. 6 represents a device for receiving and reproducing an
audio/video digital service according to the invention in which the
reproduction device estimates and compensates for a delay;

[0063]FIG. 7 illustrates a method of estimating delay for liquid crystal
screens according to the invention;

[0064] FIG. 8 illustrates a chart representing the response times of
liquid crystals for different grey transition levels;

[0065] FIG. 9 illustrates a manual delay selection method according to the
invention;

[0066]FIG. 10 illustrates a method of selecting delays for different
video formats;

[0067]FIG. 11 illustrates a manual delay estimation device according to
the invention;

[0068]FIG. 12 illustrates a semi-automatic delay estimation device
according to the invention;

[0069]FIG. 13 illustrates a semi-automatic delay estimation method
according to the invention;

[0070]FIG. 14 represents a device for receiving and reproducing an
audio/video digital service according to the invention which uses an
external audio reproduction device, the reception device including 2
delay modules; and

[0071]FIG. 15 represents a device for receiving and reproducing an
audio/video digital service according to the invention, the reception
device being connected to two video reproduction devices.

5. DETAILED DESCRIPTION OF THE INVENTION

[0072] The embodiments will be described with particular reference to an
audiovisual digital service. The A/V source is likened to a decoder but
can be any other type of A/V source (for example, a DVD player). The
audiovisual reproduction device is likened to a television including a
screen and an audio output (i.e. built-in speaker) but can also be any
other type of audiovisual reproduction device (for example a computer).
The audio reproduction device can be external to the television and
likened to a device including an amplifier linked to one or more speakers
(for example, an audio amplifier of a home cinema device) but can also be
any other type of audio reproduction device.

[0073] Some deinterlacing circuits have compensating audio inputs to which
is applied the same delay as to the video to remain in phase. However, in
the case where the user chooses to use the sound from an external audio
reproduction device (for example of home cinema type), no delay
compensation is applied. It therefore seems natural to place the A/V
synchronization module in the digital decoder, the latter being the
source of the A/V signals and moreover necessarily compatible with A/V
equipment already on the market. One of the principles of the invention
is to provide automatic means to the television so that the latter can
make known to the decoder the value of the delay between the video at the
input of the television and the video displayed on the screen.

[0074] FIGS. 2 and 3 represent two variants of a device for receiving and
displaying an audiovisual digital service according to the invention.
Only the main elements of the device are shown. FIG. 2 represents an
audiovisual service device which includes a digital decoder 20 linked to
a television 21, in particular by links 220, 221 and 222. The decoder 20
receives at its input an encoded audiovisual stream 22 (for example in
MPEG coding). This A/V stream 22 is demultiplexed by a demultiplexer 204
into at least one audio signal and one video signal. The video signal is
then decoded by a video decoder 200. As for the audio signal, this is
decoded by an audio decoder 201. The two streams are synchronized using
the A/V synchronization module 202 which communicates with the two
decoders 200 and 201. The A/V synchronization module 202 is, moreover,
linked to a processing unit 203. The video decoder 200 is linked to the
television 21 via a DVI/HDMI link 220 (DVI/HDMI standing for Digital
Video Interface/High Definition Multimedia Interface). More specifically,
the video decoder 200 is linked to a video processing module 210 of the
television 21. This processing module is in turn linked to a screen 211.
As for the audio decoder, this is linked to an audio reproduction device
of the television 212 via a link 222. The synchronization module 202 is
linked via the unit 203 to a non-volatile memory 213 (for example EDID
EPROM, in which EDID stands for Extended Display Identification Data) of
the television 21 via an I2C bus 221 using, for example, the DDC
communication protocol (Display Data Channel protocol for recovering
screen-related data).

[0075]FIG. 3 represents a similar device in which the audio stream is
reproduced by an external audio reproduction device 31 (for example an
amplifier of a home cinema device). This device 31 includes an audio
amplifier 310 linked to speakers 33. The decoder 30 includes elements (in
particular, audio decoder, video decoder, demultiplexer, A/V
synchronization module) similar to those of the decoder 20, elements
which are given the same reference numbers. It also includes a
programmable audio delay module 300, an HDMI interface 302 and a video
format management module 301. It is linked to a television 32 including a
video processing module 320 which is in turn linked to a screen 321. The
decoder is linked to the external audio reproduction device 31 via a link
340, for example via an SPDIF (Sony/Philips Digital Interface) interface
and to the television 32 via a DVI/HDMI link. The objective of the two
proposed solutions (FIGS. 2 and 3) is to compensate for a delay Ddc,
in which Ddc=Dd+Dc, induced by video processes (for
example deinterlacing, format conversion, image quality enhancement, etc)
320 210 and/or a delay De due to the screen (for example, the
response time of the liquid crystals) 211. The delay De can itself
be the sum of a number of delays such as those defined in the table
above. In the rest of the document, the overall delay, i.e.
D=Ddc+De, is denoted D. Ddc and De can be fixed or
variable in time, as is the case of the delay De for liquid crystal
screens for which the response time is variable from frame to frame. In
the case where the screen introduces no delay, D=Ddc. Similarly,
Ddu can be zero.

[0076] According to the invention, at least a programmable delay D is
applied to the audio signal by storing it either in its compressed form
or in its decoded form. The delay D is applied to the audio signal in the
module 300. According to a variant of the invention, the delay is
directly applied to the audio signal in the A/V synchronization module
202 present in the decoder. The decoder is responsible for applying the
appropriate delay value D in the audio delay module 300 or in the A/V
synchronization module of the decoder 202 to compensate for the delay
induced by the video processes and/or the delay due to the type of screen
210 320.

[0077] According to the invention, the delay D induced by the television
21 or 32 can vary according to the format of the input video, the
management of which is handled by the module 301. Thus, a new delay value
D can be programmed in the audio delay module 300 or in the A/V
synchronization module of the decoder 202 each time the video format
changes if this delay depends on the video format. This delay is denoted
Dx, where x is the video format with which the delay is associated.
The programmed delay value can also be an overall value D operating
independently of the input video format.

[0078] The proposed solution therefore consists in enhancing the HDMI/DVI
control protocols with parameters that indicate the delays Dx, for
example, for different video formats, or even the overall delay D
independently of the input video format. These protocols enable the
screens to share with the decoder information concerning their
characteristics and capabilities. According to these protocols, the video
source uses the DDC channel 221 to read a non-volatile memory 213 placed
in the television 21 or 32 to ascertain, for example, the resolution, the
polarities of the synchronization signals and colorimetric data. This
data is represented using EDID descriptors which are defined in document
EIA/CEA-861B. They can be supplied by the screen manufacturers and
programmed in the EPROM EDID memory 213.

[0079] The invention therefore consists in adding EDID information
descriptors other than those already standardized in order to store in
the television information characteristics of the delays introduced
either by the digital video processing of the television (Ddc), or
by the response time of the screen (De), or by both (D or Dx).
The delay information for each television 21 or 32 equipped according to
the invention is stored in a non-volatile memory of the television 213.
This information can include the 4 delays Dx corresponding to the 4
video formats described previously (50 Hz interlaced input, 50 Hz
progressive input, 60 Hz interlaced input, 60 Hz progressive input).
Storing the delays relating to other video formats can also be envisaged.

[0080] According to the invention, the decoder recovers these values in
order for the A/V synchronization module 202 or the delay module 300 to
synchronize the audio and video streams. The information concerning the
delays Ddc and De can be supplied by the manufacturer of the
television 21 or 32 and can be transmitted by the television 21 32 to the
decoder 20 in electronic form. The overall delay information D or Dx
must then be transferred on switching on the decoder. This information
can also, optionally, be transferred on a change of channel if necessary
or on request to the decoder.

[0081] An alternative solution to the use of the DDC channel is to use the
CEC (Consumer Electronics Control) interactive interchange protocol
specified in HDMI.

[0082]FIG. 4 illustrates a particular embodiment of the invention. The
decoder 20 is the same as the one described in FIG. 2 and will not be
described further. The television 41 includes elements similar to the
television 21 which are given the same references and will not therefore
be described further in detail. The television also includes a module 410
for estimating a delay De. In practice, for televisions which
introduce a delay that is variable in time according to the screen type,
this delay must be estimated. This estimated delay De is then added
to the delay Ddc induced by the various video processes (for
example, deinterlacing). The value of the overall delay D is stored in an
EPROM EDID memory 411 to be used by the A/V synchronization module 202 or
the delay module 300 placed in the decoder 20. This delay is therefore
recovered by the decoder via the DDC link 221 in order to synchronize the
audio and video streams. Storing the different delays (Ddu and
De) separately in the EPROM EDID memory can also be envisaged, these
different delays then being recovered by the decoder via the DDC link
221. To estimate the variable delay De due to the screen type, the
estimation method described later in the document can be used for liquid
crystal display screens.

[0083] In another embodiment represented in FIG. 5, the delay linked to
the screen De is estimated in an estimation module 500 located in
the processing unit of the decoder 50. The elements similar to the
preceding figures are given the same references and will not be described
further. According to the invention, it is therefore proposed to define
descriptors for storing, in the EPROM EDID memory of the screen, the data
needed to estimate the delay (for example, charts as supplied by the
liquid crystal suppliers and illustrated in FIG. 8). This data is then
recovered via the link 221 by the decoder which itself estimates the
average delay in the module 500. For this, it can use the estimation
method described later in the document.

[0084] One advantage of the solutions described is to synchronize the
audio and video streams when using a television and an external audio
output device 31 (for example, HiFi system, home cinema device).

[0085] In another embodiment illustrated in FIG. 6, the delay De
linked to the screen is also estimated in the block 410 of the television
61 as previously. However, the synchronization is carried out directly in
the television. In this case, a memory 610 in the television can be used
to buffer the audio data and restore it to the user according to the
average delay. A method of estimating the average delay is described
below.

[0086]FIG. 7 illustrates a device according to the invention for
estimating 7 the delay De for liquid crystal screens. The device
includes a frame memory 71 and a delay computation module 70. The frame
memory 71 is used to delay the input video (video tN) by one frame
(video tN-1). The device 7 can use the frame memory 71 of the
enhancement device. The delay computation module 70 computes, for each
pixel, the difference in grey levels between two successive frames. This
module then uses the charts (FIG. 8) supplied by the liquid crystal
manufacturers. These charts give the response times for different grey
level transitions. They can thus be used to estimate, for each pixel, the
response time between two frames. By constructing a histogram on all the
pixels, an average delay De is therefore estimated (for example, by
calculating a weighted average which takes into account the number of
pixels having a given response time) which is used by the A/V
synchronization device. In the case where the estimation of the delay
De is performed in the decoder (FIG. 5), these charts can be stored
in the EPROM EDID memory and recovered by the decoder via the DDC link
221. In this case, the delay estimation device 7 then recovers the EDID
data via the link 72.

[0087] The following solutions propose other embodiments for manually or
semi-automatically making known to the video source the delay parameters
D induced both by the video processes and by the screen. These solutions
are in particular used when there is no HDMI link between the A/V source
and the video reproduction device.

[0088] FIG. 9 illustrates a manual tuning method enabling the user to
select delay parameters empirically using a menu. The selection device is
shown in FIG. 11. This method is useful when no delay information is
supplied by the manufacturer. In this case, the decoder 110 generates an
A/V sequence enabling the user to synchronize the delay D manually and
finely. According to the invention, in a step 91, the decoder 110
switches to the appropriate video format (denoted X, for example 50 Hz
interlaced). In a step 92, the user 115 then selects a delay value
Dx from the menu for this format. The value is programmed in the
audio delay module of the decoder 300 or in the A/V synchronization
module 202. In a step 93, the decoder then sends the synchronized A/V
sequence 111 using the selected delay value. The video is then displayed
on the screen 112. The synchronized audio signal is amplified by the
audio amplifier 113 and the sound is reproduced using speakers 114. The
user can judge the quality of synchronization by watching the screen 112
and listening to the sound produced by the speakers 114. In a step 94,
the user informs the decoder using the menu as to whether the
synchronization is accurate enough for him. If not, the process is then
repeated with a new delay value D. If the synchronization is
satisfactory, the selection is ended and the value of Dx is stored
in the programmable delay module 300 or in the A/V synchronization module
202.

This operation can be repeated for any video format in order to
determine, for each, the delay that should be applied to the audio
stream. This method is illustrated in FIG. 10. The delays D251
(relating to the 50 Hz interlaced video format), D50p (relating to
the 50 Hz progressive video format), D30i (relating to the 60 Hz
interlaced video format) and D60p (relating to the 60 Hz progressive
video format) determined, in the steps 101, 102, 103 and 104
respectively, are selected in turn according to the method described in
FIG. 9.

[0089] According to another embodiment, since the delay is known because
it is, for example, supplied by the manufacturer, the user can use a menu
to manually enter the delay values Dx to be applied in the delay
module 300 or in the A/V synchronization module 202 of the decoder for
example for different video formats. These values may be, for example,
entered on installing the digital service device.

[0090] According to another device illustrated in FIG. 12, a probe 122 is
used, fixed to the screen in order to detect characteristics of what is
displayed on the screen and return this information to the decoder. Such
a semi-automatic method of estimating the delay D is illustrated in FIG.
13.

[0091] According to this method, the decoder 120 generates a series of
black images 130 (i.e. of low grey levels), then a single white image 131
(i.e. of high grey levels) and a series of black images 132 again, which
it sends to the television 121. The first series of black images is then
displayed 133 on the screen, followed by the white image 134 and finally
the second series of black images 135. The probe 122 is capable of
detecting a white image on the screen and of sending an instantaneous
message to the decoder 120 to inform it of the display 134 of this white
image. The decoder 120 computes the time elapsed between the moment 138
at which the white image was sent by the decoder and the moment 139 at
which it was displayed on the screen of the television 121. The probe 122
is typically a device that is sensitive to light intensity and that can
be positioned against the screen, for example in the top left corner of
the screen or even in the middle of the screen. It is, moreover, capable
of instantaneously evaluating the light intensity over a limited area of
the screen. The probe 122 has 2 logic states. It is in a first state 136
when the light intensity level detected is below a certain threshold
(i.e. on displaying the first series of black images) and in a second
state 137 when the level is above the threshold (i.e. on displaying the
first series of white images). The threshold is determined such that,
when a black image is displayed, the probe is in the first state and,
when a white image is displayed, the probe is in the second state. The
black image can be replaced with an image of low light intensity and the
white image with an image of high intensity. All that is needed is for
the probe to be capable of detecting the transition from one to the
other. Its logic state can be translated into an electrical signal having
2 values. This signal can then be recovered by the decoder. The decoder
stores the time 138 from which the white image starts being sent to the
screen and stores the time 139 at which the probe detects the transition
from the first state to the second state. The difference D 140 between
these 2 time markers represents the delay induced by the video processes
and the screen. This operation can be repeated for various video formats
in order to obtain a set of delays Dx for all the video formats.
This method is semi-automatic since, even if no menu is used, the user
must connect the probe to the decoder, apply it to the screen and begin
the process manually.

[0092] It is also possible to envisage having a blue screen with a black
square displayed somewhere on this screen. With the probe 122 positioned
on the black square, white is sent to the black square in order for the
probe to detect the change of light intensity. Knowing the position of
the probe on the screen means the delay can be measured more precisely
for screens that do not reproduce all the pixels simultaneously (for
example, for scanning screens).

[0093] Another application of the present invention is disclosed in FIG.
14. The elements common to FIGS. 2 and 3 are given the same references
and will not be described further. The decoder 140 includes an additional
programmable audio delay module 141. This second module is used to
synchronize the audio and video streams that will be reproduced by the
same reproduction device 142 (for example a television). This television
142 includes a video processing module 320 which induces a delay Ddc
on the video. It also includes an audio processing module 144 which
induces a delay Dta on the audio stream. This module is in turn
connected to speakers 151 built into the television. In order for the
audio and video streams reproduced by the built-in speakers 151 and the
screen 321 to be synchronized, the second programmable audio delay module
141 applies to the audio stream output from the decoder a delay
(Ddc-Dta). In this example, it is proposed to add to the EDID
table 4 parameters relating to the video delays Ddc introduced by
the reproduction device which depend on the video format:

[0094] 50 Hz
interlaced input

[0095] 50 Hz progressive input

[0096] 60 Hz interlaced
input

[0097] 60 Hz progressive input 4 parameters relating to the delay
Dta introduced by the audio processing system for the above 4 video
formats are also added. The delay values can be encoded on 1 byte in
order to represent delay values varying from 0 to 255 milliseconds. Thus,
in the case where the audio stream is reproduced by an external device 31
(for example SPDIF), the source applies a delay Ddc (module 300) to
the audio stream at the output of the decoder 201. In the case where the
audio stream is reproduced by the HDMI reproduction device (for example a
television) 142, the source 140 applies to it a delay
(Ddc-Dta), via the module 141, which takes account of the video
and audio delays introduced by the different processing systems of said
HDMI device 142.

[0098] A different application is illustrated in FIG. 15. The elements
common to FIGS. 2 and 3 are given the same references and will not be
described further. The decoder 150 is connected to two televisions 152
and 155 located in two different rooms. The television 152 is linked to a
home cinema type device 31 for audio output (for example, via an SPDIF
interface). The television 155 which mainly includes a screen 157 and
which is located in another room, receives the audio and video streams
together via a link 162 (for example scart or analogue RF). The
television 152 includes a video processing module 153 introducing a delay
Ddu and a screen 154. As for the television 155, it maintains the
synchronization of the audio and video streams that are transmitted
together to it. According to the invention, a device is proposed that has
a single audio output which is synchronized with any one of the video
outputs. The solution according to the invention consists in
synchronizing, in the decoder, the audio stream with the first video
output 158 as already described with reference to FIG. 3 (that is, by
applying a compensating delay to the audio). A second programmable delay
module 161 is added which will apply to the second video output 159 the
same delay D=Ddc as to the audio output in order for the second
video output 159 to be synchronized with the audio output 160. This
solution in particular makes it possible to use the second television 155
in another room with the audio output of this same television and to have
only one and the same audio output 160 operating with both video outputs
158 and 159.

[0099] The invention described in the context of the DVI and HDMI
communication protocols could be extended to any control protocol
developed in the future, provided that it allows for the interchanging of
such delay data or data for computing delays in the decoder (for example,
charts).

[0100] The invention is disclosed in the context of the synchronization of
audio and video streams of a digital service, in the case where the
screen introduces a delay whereas the audio part of the digital service
is associated with an instantaneous process. It can be generally applied
to any type of device for reproducing any digital service, said service
being separated into different parts processed by different reproduction
devices, each of them applying specific delays to the part of the service
that it handles. In this case, the capability of communicating the
specific delay of the reproduction device to the source device enables
the latter to synchronize all the parts of the digital service between
themselves for good reproduction of the complete service.